Maybe a wiring diagram would help. Here is a 1964 Chevy truck wiring diagram I found. (link to 67-72chevytrucks.com)
Just because the front turn lights work correctly doesn't necessarily rule out the turn signal switch. Reconnect the wires in the rear and disconnect the turn signal switch column connector. Using a jumper wire, power each of the rear turn wires (gReen is R and yeLLow is L) one at a time. If the lights work independently when powered directly from the turn signal switch harness connector, then the problem is in the switch itself.
If the both rear turn lights light up when you power only the green or yellow wire at the turn signal switch connector, that would confirm the problem is in the wiring.
I'm sure there are many ways to track this problem down. Hopefully you'll get some input from others here as well.